1. Field of the Invention
This invention relates to high-voltage insulation for electric conductors especially for the winding bars and coils of electric machines by wrapping mica tape around the electric conductor and impregnating the wrapped conductor with epoxy resin, and to the mica tape for wrapping the electric conductor.
2. Background of the Invention
In insulation technology, especially in connection with the high-voltage insulation, to be impregnated with hot-setting epoxy resin, of electric machines and apparatus, the problem of a storable and flexible synthetic-resin system which can be used at the same time as tape adhesives for wrapping tapes and as accelerators for the subsequent epoxy resin hardening exists. The high-voltage windings of electric machines and apparatus, including particularly turbo-generators, transformers or parts thereof, are wrapped for insulation with mica tape. For impregnating the wrapped or bandaged parts, large tanks with impregnating resins are used, into which the products to be manufactured are immersed. The quantity of resin consumed is small as compared to the quantity in the impregnating tank; it is replaced when necessary to replenish the resin. In this connection it is of great importance that the resin in the impregnating tank retain its properties as long as possible, including its viscosity, so that the same impregnating conditions can be used for every product to be impregnated, i.e., that the maintenance of the constant properties of the hardened resin does not take place at the expense of the desirable or applicable setting conditions. As the resin-hardening system, a hot-setting epoxy resin of the bi- or higher-functional glycidyl ether type or of the epoxy compound and acid anhydride type, the viscosity of which is less than 30 cP (at about 60.degree. to 70.degree. C.), is preferably used.
In insulation technology, the accelerator for the hardenable resin is incorporated for this purpose, according to the present state of the art (German published prosecuted application No. 1 219 554, German published prosecuted application No. 11 62 898), either by immersion of the carrier material into a solution of the accelerator or by incorporating the accelerator into the binder which acts as the adhesive between the top and bottom side of the wrapping tape. The accelerators incorporated into the adhesive resin are either accelerator resins of the epoxy resin type with tertiary-bound amine nitrogen, which serve at the same time as adhesive resins, or conventional epoxy resin hardening-accelerators of the tertiary amine type such as 4,4'-dimethyl-amino diphenyl methane (T-base) which are embedded in the adhesive resin.
The disadvantage of these depot resin systems is in the time-limited storage stability, caused by anionic polymerisation of the adhesive resin by the embedded tertiary amine. This polymerisation leads to hardening which impairs the wrappability of the insulating tape. With the accelerator resins of the epoxy resin type with tertiary-bound amine nitrogen, which are used in situ, application over an area is furthermore not possible because these resins are solid resins, so that impregnating the tape with the low-viscosity impregnating resin mixture of the epoxy resin type is difficult. It has been attempted to counter this difficulty by applying the accelerator resin spot-wise in an amount just sufficient for cementing (German published prosecuted application No. 2 215 206).